I don’t know if anyone has posted anything like this before or not…
I think that one must be dead before one can be infected/possessed.
We all know that Sayid was dead before he became infected, but I believe this is the case for Claire and Christian Sheppard as well. I have long held the belief that Claire died in that explosion in Dharmaville which is why she started acting weird and why Miles was staring at her so strangely at the time. I also believe that Christian is infected as well which is why his body wasn’t in his casket when Jack found it. Some people may not think that Christian is infected and that MIB just appeared in the form of Christian, but I think that Claire saying her friend and her father both told her the others had her baby is proof enough that MIB is not posing as Christian.
The only person that we know was infected, but don’t have any idea if they died was Roussaeu. I think in this case however, we haven’t been shown too much of her life before she was infected, so she could have died at some point before she was infected, maybe even when they were first ship-wrecked.
The only problem I have is how to kill the zombies. Obviously Roussaeu was killed, but at the temple Dogen said that Sayid had to die willingly. My only thoughts are that the others have to obey whatever rules of freewill that exist, i.e. can’t directly kill the zombies of MIB, but people outside of their circle do not, i.e. the mercenaries from the freighter that killed Roussaeu.